Typical prior art flat panel liquid crystal display systems are described in "Flat-Panel Displays Come on Strong in Speed, Resolution and Color," Computer Design, Feb. 1, 1989, pages 65 through 82. Operation and performance of direct multiplexed liquid crystal displays, including the twisted nematic (TN), the supertwisted birefringence effect (SBE), and the surface-stabilized ferroelectric liquid-crystal (SSFLC) display, are described by Scheffer "Direct-Multiplex Liquid-Crystal Displays," Seminar 4, Society for Information Display (SID) International Symposium Seminar Lecture Notes, Vol. 1, May 11, 1987, pages 4.1 through 4.34. U.S. Pat. Nos. 2,400,877; 2,481,380, and 2,544,659, issued to J. F. Dreyer pertain to the use of an aligned organic dye as a polarizer.
It is known in the art to combine fiber optic faceplates with LCD systems. An example of such a combination relevant to this present invention is described in U.S. Pat. No. 4,349,817 to Hoffman et al. (Hoffman), which patent is hereby incorporated by reference into the present case. Major advantages of LCDs include their compact, rugged construction and their portability as display screens for portable personal computers.
Generally, LCDs which are intended for use in portable systems are of the reflection type in order to make use of available ambient light for illumination rather than incurring the weight, bulk, and power consumption characteristic of active backlighting. Such displays include a liquid crystal layer which is sandwiched between transparent front and back electrodes, and a specular or semi-specular (i.e., mirror-like) surface placed behind the display to enhance reflection. The system has an off-state, i.e., no voltage is applied between the front and back electrodes, and an on-state, i.e., such a voltage is applied.
The Hoffman patent pertains exclusively to LCDs of the dynamic scattering type. When this type of LCD is in the off-state the liquid crystal is clear, permitting light to pass through and be reflected back out by the reflective back electrode. In the on-state the liquid crystal scatters light increasingly in proportion to increasing applied voltage. This mode of controlling the light transmissivity of the liquid crystal material in response to the applied voltage is called the "dynamic scatter mode" (when light is transmitted through the LCD) and the "reflective dynamic scatter mode" (when light is reflected out the same side of the LCD).
A major problem with the reflective dynamic scatter mode LCD device used for direct viewing is that of contrast, defined here as the brightness ratio of the on-state to the off-state. The problem with this particular art, then, is how to reduce excessive levels of incident light emanating from unwanted light sources which are positioned outside the viewing angle of the LCD screen. Contrast desirably increases if these unwanted light sources can be neutralized.
To solve this contrast problem resulting from stray light, Hoffman coupled a specially designed fiber optic faceplate to a LCD system. To describe Hoffman's approach and the state of the prior art, applicants include FIG. 1 and FIG. 2, labelled as Prior Art in the present case. These figures plus the discussion below paraphrase the disclosures surrounding Hoffman's respective FIGS. 3 and 2.
FIG. 1 (Prior Art) shows a direct view liquid crystal image display system 20 viewed directly under ambient light from a source of light 22 such as a bright sky or brightly lit room (not shown). Light rays 24, 25, and 26 from light source 22 hit a fiber optic faceplate 28 within an acceptance cone Theta.sub.max [herein, T(max)] of faceplate 28. Rays 24, 25, 26 are transmitted to a liquid crystal layer 30 of a liquid crystal display device 31 over which faceplate 28 lies.
The definition and significance of the acceptance angle T(max) appears in the discussion about Equation (1) below. T(max) is measured with respect to an axis 27 which is parallel to the horizontal light-propagating axis (not labelled) of the optical fibers comprising fiber optic faceplate 28 and perpendicular to the face of faceplate 28.
First considering the off-state condition, ray 24 hitting a localized liquid crystal area 32 when in the off-state is specularly reflected along a ray path 34 to the eye 36 of an observer (not shown) who as a result sees a bright display region.
Conversely, now considering the on-state condition, previously mentioned light ray 26 hits a localized liquid crystal area 38 which is in the on-state, with the result that ray 26 is scattered so that only a portion of ray 26 is reflected. That is, the reflected portion of the scattered light follows a path 40, 42, 44, 46, and 48 back to observer eye 36, which thus sees a relatively dark display region 38 (i.e., on-state region 38).
To address stray light coming from other light sources such as those positioned as are light sources 50, 52, and 54 (our Sun), existing technology configures system 20 so that light enters and leaves faceplate 28 only within a well defined faceplate 28 acceptance angle T(max). By this approach, stray light is absorbed by faceplate 28.
That is, by absorbing light from sources outside the acceptance cone T(max) of faceplate 28, such as a ray 56 from the Sun 54 and a ray 58 from light source 52, faceplate 28 prevents undesirable loss of image contrast of the LCD images with respect to ambient light generated by such light sources as 22, 50, 52, and 54.
FIG. 2 (Prior Art) illustrates in cross-section a single optical fiber 60 of the type bundled together to make up faceplate 28 shown in FIG. 1. Faceplate 28 is formed with many parallel optical fibers 60 which are fused together. Each fiber 60 has a light-transparent core 62 having an index of refraction n.sub.1, covered with a light-transparent sheath 64 having an index of refraction n.sub.2 which is less than n.sub.1, which in turn is covered with an optically absorbing material 66.
Faceplate 28 has an acceptance cone of angle T(max), an angle related to the index of refraction=n.sub.1 of core 62 with respect to the index of refraction=n.sub.2 of sheath 64. These attributes are related according to the well-known relationship expressed in Equation (1) below: EQU sin T.sub.max =[(n.sub.1)2-(n.sub.2)2].sup.1/2 =N.A. (1)
where N.A.=the Numerical Aperature of the optical fiber.
An incident light ray 25 falling within the acceptance angle T(max) to optical fiber axis 27 propagates through core 62 by the well known phenomenon of multiple total internal reflections from a boundary 70 existing between core 62 and sheath 64. Conversely, an incident light ray 58 falling outside incidence angle T(max) will not be totally reflected, but instead will propagate through boundary 70 into transparent sheath 64, finally to be absorbed by light absorbing layer or material 66.
More simply stated, the function of the fiber plate in the Hoffman LCD is to absorb all light which strikes the display outside the viewing angle of the display (defined as the angle over which the display provides an image of acceptable contrast), thereby reducing stray light and enhancing the contrast of the display. Even with this enhancement, however, the dynamic scattering type of LCD has not become a commercially important device due to its relatively limited viewing angle and poor contrast.
Twisted nematic (TN) and super twisted nematic (STN) LCDs, on the other hand, have become commercially important in the last 10 or so years largely because they offer improved contrast and viewing angle compared to previous types, such as the dynamic scattering LCD with or without the Hoffman improvements.
Limited contrast and viewing angle, however, remain among the most serious shortcomings of TN and STN LCDs, improved in these areas though they may be. The LCD industry, in fact, continues to seek displays capable of delivering the general appearance of printed characters on paper.
Application of the teachings of Hoffman will not improve, and in fact will seriously degrade, the contrast of a TN or STN LCD. This is for two reasons:
First, eliminating light which strikes the display at angles to the display surface normal greater than the viewing angle of the display will not enhance the contrast because TN and STN displays depend upon the action of polarizers on polarized light propagating within the display rather than scattering to produce the light and dark areas of their images.
Second, introduction of a fiber plate, as taught by Hoffman, in near-contact with the liquid crystal layer itself will seriously reduce the image contrast because light passing through such a fiber plate is strongly depolarized, thus largely destroying the distinction between the light and dark areas of the image.
Additionally, it is neither necessary nor desirable to incorporate a means, such as the Hoffman style fiber plate, which absorbs all light outside of the nominal viewing angle of the display into a TN or STN LCD.
The black interstitial material in the Hoffman fiber plate causes a sharp transition from a normal display appearance to a completely black display appearance with increasing angle, which can be annoying to the viewer of a TN or STN type LCD. This is because the contrast of TN and STN LCDs degrades slowly with angle, and, although viewing contrast may not be fully acceptable at high angles, a viewer may be able to determine the general nature of what is being displayed or merely that something is being displayed, even when he views the display at high angles.
Since this information is frequently important or desirable to the viewer, the Hoffman style fiber plate does not constitute an improvement to many modern types of LCDs.
Hoffman teaches the application of a fiber plate composed of fibers each having as low numerical aperture as possible and restricting the viewing angle by means of the black interstitial material as much as possible in order to reject as much stray light as possible. In contrast, the present invention teaches the application of fiber plates having as high a numerical aperture as possible and permitting as wide a viewing angle as possible in order to gather as much ambient light as possible to illuminate the display.
FIG. 3 shows a typical prior art reflective liquid crystal display utilizing polarizers. LCD 300 includes a layer of liquid crystal material 301 sandwiched between drive matrices 302a and 302b for applying electric fields to appropriate locations within the layer of liquid crystal material 301. For example, selected pixel 304 (shown as dark, but may actually appear dark or light depending upon polarizer orientation) is shown within the layer of liquid crystal material 301, caused by an appropriate electric field in that location applied across that portion of liquid crystal material 301 by matrices 302a and 302b.
Glass plates 303a and 303b serve to support drive matrices 302a and 302b. On the other sides of glass plates 303a and 303b are formed polarizers 305a and 305b, respectively. Polarizer 305a serves as the exposed surface of liquid crystal display 300, and polarizer 305b faces semi-diffused mirror 307, separated from polarizer 305b by gap 306 which may be conveniently filled up by a glass plate. If desired, mirror 307 is formed as an aluminized coating on the surface of polarizer 305b which is not in contact with glass plate 303b.
One of the disadvantages of the prior art liquid crystal display 300 of FIG. 3 is that, since glass plate 303b is generally rather thick compared with the pixel-to-pixel spacing, and since mirror 307 is specular or semi-specular in nature, a ghost image or "shadow" 310 is formed below the actual pixel 304. A simple construction of the paths of two light rays according to well-known principles of geometrical optics suffices to show that this is true. Consider ray 308-1 which enters the display from above on the left, traverses the display cell, is reflected by mirror 307, and reemerges from the display as ray 308-2. Consider also ray 309-1 which enters from above on the right and reemerges in a similar manner as ray 309-2. Extensions of rays 308-2 and 309-2 cross at 310, and hence appear to an observer to have come from 310. Also, since both rays pass through the location of the selected pixel 304, the intensity of both rays is modulated by the action of the display to be the same as that of the selected pixel 304. Ghost image 310 is thus a virtual image in the geometrical optics sense of the selected pixel 304 lying behind 304 and, depending upon the observer's viewing position, may appear laterally displaced from 304 as well due to viewing parallax.
An additional disadvantage of prior art liquid crystal displays such as 300 in FIG. 3 is that the apparent illumination of the display is a strong function of viewing angle, and that the display appears most strongly illuminated by ambient light when viewed at an angle close to that at which light also specularly reflects from the top display surface (the top surface of polarizer 305a in FIG. 3). The viewer is thus frequently tempted to view the display in a manner which causes him to have to contend with annoying glare.